Threshold voltage extracting method and circuit using the same

Abstract: The transistor threshold extraction circuit in accordance with the present invention has an output (OT) and comprises: a) at least two transistors (M1, M2) of the same type having respectively two control terminals (G1, G2) and having essentially the same threshold with each of said two transistors (M1, M2) also having a first (S1, S2) and a second (D1, D2) main conduction terminal, b) a current mirror (MC) having at least two input-output terminals (IM, OM) with said two terminals (IM, OM) coupled respectively to said two transistors (M1, M2) so as to supply to them the bias currents, c) a voltage generator (VG) connected between said two control terminals (G1, G2), and d) a feedback path (FP) between said control terminals (G1, G2) and one (OM) of said input-output terminals. The output (OT) is coupled to one (G2) of said control terminals.

Description: The present invention relates to a transistor threshold voltage extraction method in accordance with the preamble of claim 1 and a circuit in accordance with the preamble of claim 4. Threshold voltage extraction finds various applications in the field of the characterisation of electronic devices, level translation, absolute or relative temperature measurement, temperature compensation, and compensation of process parameters. A specific panorama of this subject is set forth in the article by Zhenhua Wang, "Automatic Vt Extractors... and Their Applications", in IEEE Journal of Solid-State Circuits, Vol. 27 No. 9 pages 1277-1285, September 1992. This article makes known the circuit shown in FIG. 1 annexed hereto. It comprises two N-channel MOS transistors M1 and M2 having the same threshold voltage and a current mirror MC having an input terminal IM and an output terminal OM. The source terminals S1 and S2 of the transistors M1 and M2 are connected to a ground terminal GND, their drain terminals D1 and D2 are respectively connected to the terminals IM and OM, and their gate terminals G1 and G2 are respectively connected to the input IT and output OT. In addition the gate and drain terminals of the transistor M2 are connected together. The potential at the output OT is given by a linear combination of the input potential IT and the threshold voltage of the transistors M1 and M2. This depends only on geometric parameters with the exception however of the potential at the input IT. Again, said article proposes a variation of the circuit mentioned above in which by selecting the ratio W/L of the transistor M1 equal to one fourth of the ratio W/L of the transistor M2 and connecting to the output of the above circuit an amplifier with gain of two, there is achieved at the output a potential equal to the sum of the potential at the input IT and of the threshold voltage of the transistors M1 and M2. Said circuits have the advantage of extracting the threshold voltage free from body effect since the source terminal of the N-channel transistors is connected to the substrate (in the case of N-well process) or to the process well (in the case of P-well process). Other circuits require having separate wells in which to insert the transistors which are desired free of body effect, or limitation of threshold extraction to transistors of a single polarity. A known prior art solution is disclosed in the IEEE Journal of Solid State Circuits, Vol. 23 No. 3, June 1988 N.Y.) which relates to a current reference circuit. Such a circuit includes a first and a second transistors and a voltage generator which is connected between the gate of the first transistor and the gate of the second transistor. However, such a reference circuit cannot be used as a threshold voltage extraction circuit. The purpose of the present invention is to supply an alternative circuit to that of the known art. Said purpose is achieved by means of the method having the functions set forth in claim 1 and by means of the circuit having the characteristics set forth in claim 4 while additional advantageous aspects of the present invention are set forth in the dependent claims. By using a voltage generator and connecting together the control terminals of the two transistors through at least said generator and determining a feedback path between the control terminals and one of the input-output terminals of the current mirror there are achieved the same advantages as those of the circuit of the known art. Advantageously having created a closed feedback loop (having gain less than one) the operation of the circuit is more constant. In addition, at the output of the circuit in accordance with the present invention it is possible to achieve a potential equal to the sum of the threshold voltage and the generator voltage multiplied by a constant which depends only on geometrical parameters. Said addend can thus be controlled very well whether large or small values are desired. The present invention also relates to a circuitry system using and comprising a circuit in accordance with the present invention for operation independently and/or dispersion parameters having the characteristic set forth in claim 12. The present invention is disclosed by the description given below considered together with the annexed drawings in which: FIG. 1 shows a circuit in accordance with the known art, FIG. 2 shows a first circuit wich helps to understand the present invention, FIG. 3 shows a second circuit wich helps to understand the present invention, FIG. 4 shows a first circuit in accordance with the present invention, FIG. 5 shows a second circuit in accordance with the present invention, and FIG. 6 shows a thershold extractor circuit in accordance with the present invention and a second stage. The circuit of FIG. 2 comprises N-channel MOS transistors M1 and M2 having essentially the same threshold voltage and a current mirror MC having an input terminal IM and an output terminal OM. It has an output OT. The source terminals S1 and S2 of the transistors M1 and M2 are connected to a ground terminal GND while their drain terminals D1 and D2 are connected respectively to the terminals IM and OM and their gate terminals G1 and G2 are connected respectively to the positive and negative terminals of a voltage generator VG. In addition the gate and drain terminals of the transistor M2 are connected together by means of a feedback path FP consisting of a short circuit. The output OT is connected to the terminal G2. In FIG. 2 is also shown a very simple implementation of the mirror MC. This consists of two P-channel MOS transistors M3 and M4. Their source terminals are connected to a supply terminal VDD, their gate terminals are connected together, the drain terminal of the transistor M3 is connected to the terminal OM of the mirror MC and the drain terminal and the gate terminal of the transistor M4 are connected together to the terminal IM. The potential at the output OT is given by the sum of the threshold voltage of the transistors M1 and M2 and of the voltage of the generator VG multiplied by a constant as follows: A * K 2/ K 1 1- A * K 2/ K 1 where A is the current gain between input and output of the mirror MC, and K1 and K2 are the ratios W/L respectively of the transistors M1 and M2. This constant depends only on geometrical parameters and can thus be well controlled and made either very large or very small depending on requirements. Preferably this circuit is sized in such a way that the MOS transistors are operated normally under saturation conditions. It is recalled that in a first approximation the current of an MOS transistor in saturation does not depend on its voltage VDS. An implementation of the generator VG a bit complicated but having excellent performance in terms of independence from temperature and from the process can be obtained by modifying the circuit illustrated in FIG. 5 of the article by Zhenua Wang "A CMOS... Analog Multiplier..." in IEEE Journal of Solid-State Circuits, Vol. 26 No. 9 pages 1293-1301, September 1991. If in this circuit the terminal VSS is connected to ground, one of the two identical output stages generating a floating voltage VB is eliminated and the negative pole of the input VY is connected to ground and the positive pole is connected to a constant potential generator referred to ground, e.g. the "band gap" type, at the output VB is obtained a constant voltage not referred to ground, i.e. floating. FIG. 3 shows a circuit analogous to that of FIG. 2 but based on two transistors M5 and M6, again of the MOS type but P channel. In this case the source terminals of the transistors are connected to a supply terminal VDD. FIG. 4 shows a first circuit in accordance with the present invention wherein in particular the path FP consists of a transistor and a two-terminal circuit element and the generator VG consists of the same two elements with the addition of a current generator. There are also shown by generalities three different potential references indicated by P1, P2, P3. The terminals S1 and S2 are connected to the third reference potential P3 which can coincide in a particular case with the ground terminal GND. If this potential does not coincide with the ground, a threshold subject to body effect will be extracted. The circuit comprises an N-channel MOS bias transistor MB having its drain terminal DB connected to the second gate terminal GB connected to the terminal OM and its source terminal SB connected to the terminal G1 and comprises a two-terminal circuit element B1 connected between the terminals G1 and G2 and a current generator IG connected between the terminal G2 and the first reference voltage P1, e.g. the ground terminal GND. Ignoring the gate current of the transistors M1 and M2 the generator IG causes a constant current to flow in the transistor MB which holds it in saturation and in the two-terminal circuit element B1 which involves a constant potential difference between the terminals G1 and G2. Since the transistor MB is held in saturation the potentials of the terminals G2 and D2 are mutually interlocked. The two-terminal circuit element B1 can be provided by a resistor in a very simple manner or by diode-connected MOS transistors, by true diodes, etc.. It may happen that this circuit exhibits two work points, e.g. if the voltage falling at the ends of the two-terminal circuit element B1 is lower than the threshold. In these cases a start-up circuit is required to carry the circuit into the desired work point after starting. This is common practice in self-biasing circuits like this one. FIG. 5 shows a very advantageous variation of the circuit of FIG. 4 differentiated by the presence of a second two-terminal circuit element B2. This circuit also comprises a second two-terminal circuit element B2 essentially equal to the two-terminal circuit element B1 inserted between the terminal G2 and the generator IG. The output OT is connected to the node connecting the second two-terminal circuit element B2 and the generator IG. The generator IG causes the same current to flow both in the two-terminal circuit element B1 and the two-terminal circuit element B2 and, since these are essentially equal, at their ends is established essentially the same potential difference. The output is thus at a potential equal to the threshold of the transistors M1 and M2. Said threshold is extremely accurate because the effects of the two two-terminal circuit elements compensate for each other. In an integrated embodiment the technological and geometrical equality is relatively easy to provide. Said circuit can also be described differently by stating that it comprises a voltage divider VD having an intermediate output E3, a first terminal E1 and a second terminal E2 and consisting of two essentially equal two-terminal circuit elements B1 and B2 and stating that the output E3 is connected to the terminal G2, the terminal E1 is connected to the terminal G1 and to the terminal SB, and the terminal E2 is connected to the generator IG and the output OT. The two two-terminal circuit elements B1 and B2 can also be unequal. In this case however it must be provided that: A * K 1/ K 2 1- A * K 1/ K 2 = Z 1 Z 2 where Z1 and Z2 are the impedances of the two two-terminal circuit elements B1 and B2. A last circuit in accordance with the present invention is shown in FIG. 6. This consists of a threshold extractor circuit TE like one of those described above and of a stage having an input connected to the output OT and having an output UT of its own. This stage is identical to the extractor circuit of the known art shown in FIG. 1. This comprises two N-channel MOS transistors M7 and M8 having the same threshold voltage as that of the transistors M1 and M2 and another current mirror MC2 having an input terminal IM2 and an output terminal OM2. This is equipped with an input connected to the output OT and an output UT of its own. The source terminals S7 and S8 of the transistors M7 and M8 are connected to the ground terminal GND, their drain terminals D7 and D8 are respectively connected to the terminals IM2 and OM2, their gate terminals G7 and G8 are connected respectively to the input OT and the output UT. In addition the gate and drain terminals of the transistor M8 are connected together. If the circuit of FIG. 2 is used as the extractor circuit by choosing, e.g the gain of the mirror MC2 approximately unitary and indicating by K7, K8 the ratio W/L respectively M7, M8, the potential at the output UT is given by the sum of the threshold voltage (only one for the four transistors) and the voltage of the generator VG multiplied by a new constant having the value: A * K 2/ K 1 1- A * K 2/ K 1 = K 7 K 8 This new constant depends only on geometric parameters and can thus be controlled and made either much greater or much smaller than the old constant depending on requirements. Naturally one could connect one or more of such stages in cascade depending on the value of the desired constant. In the foregoing description reference is often made to direct connections between the various circuit elements but it will be clear to those skilled in the art that fairly often indirect connections, i.e. intermediated by other circuit elements, can also be referred to as "couplings" could be used with no effect on the operation of the related circuits. The above described circuits serve to extract the threshold of N-channel MOS transistors. If it were necessary to extract the threshold of P-channel transistors it would be necessary to use dual circuits. Some examples of said duality are that the ground terminals GND must be replaced by supply terminals VDD, the supply terminals VDD by ground terminals GND, the N-channel transistors by P-channel transistors, the P-channel transistors by N-channel transistors, etc. The circuit of FIG. 3 is e.g. the dual (in the above sense) of the circuit of FIG. 2. It is also possible to use, instead of the MOS transistors, other types of transistors, e.g. the BJT type. In this case however the threshold concept is less accurate and could correspond to that voltage established between base and emitter. The embodiments described above can be brought back to an instruction of the methodological type which consists essentially of using a circuit of the type shown in FIG. 1 and in the use of a voltage generator connected between the gate terminals of the transistors M1 and M2 (and not to ground). In accordance with another aspect there is created a closed feedback loop (instead of open) having gain less than one for stability. In accordance with another aspect, to the two transistors are applied two different and well controlled voltages. Lastly, as mentioned above, the present invention finds advantageous application in a circuit system for operation independently of temperature and/or dispersion of process parameters. Such a system comprises: a) an operating circuit block, b) at least one threshold extraction circuit in accordance with the above description and having an output, and c) at least one bias network connected at input to said output and connected at output to said block to supply bias currents and/or voltages. The purpose of such a bias network is to generate a bias current or voltage linked to the threshold of a reference element. Assuming that the threshold has a value which depends on a physical parameter and assuming that block operation also has an analogous dependence on the same parameter, by acting on the bias currents and/or voltages applied to the block in relation to the value of said threshold it is possible to compensate for the variations of said parameter (in time or from device to device) to achieve constant block operation. These types of bias networks are well known in the literature and in any case within the ability of the average technician. An example of a voltage supply circuit is found in the article of M. Sasaki and F. Ueno, "A Novel Implementation of Fuzzy Logic Controller Using New Meet Operation", in Proceedings of the THIRD IEEE INTERNATIONAL CONFERENCE ON FUZZY SYSTEMS, Vol. III, pages 1676-1681, 26-29 June 1994.

Claims: Method of determining a transistor threshold extraction by using a current mirror (MC) having at least two input-output terminals (IM, OM), at least two transistors (M1, M2) of the same type having respectively two control terminals (G1, G2) and having essentially the same threshold voltage to be determined by the method, said current mirror (MC) supplies to said two transistors (M1, M2) respectively through said two input-output terminals (IM, OM) the bias currents, said method comprising following steps: providing a voltage generator (VG) to connect together said two control terminals (G1, G2), and establishing a feedback path (FP) between said control terminals (G1, G2) and one (OM) of said input-output terminals of the current mirror circuit; and characterised by the further steps: arranging said voltage generator (VG) with a two-terminal circuit element (B1) and a bias network (MB, IG, GND) imposing on said two-terminal circuit element (B1) a constant predetermined current (IG); detecting a value of the potential on one (G2) of said control terminals (G1, G2); and determining said threshold voltage using the potential of the one of said control terminals. Method in accordance with claim 1 in which said mirror (MC), said transistors (M1, M2) and said voltage generator (VG) constitute a closed feedback loop between the current mirror output (OM) and input (IM) terminals, said feedback loop having gain less than one. Method in accordance with claim 1 in which to the gate terminals of said two transistors (M1, M2) are applied two different control voltages. A transistor threshold voltage extraction circuit having one output (OT) and comprising: a) at least two transistors (M1, M2) of the same type having respectively two control terminals (G1, G2) and having essentially the same threshold voltage and each of said transistors (M1, M2) having also a first main conduction terminal (S1, S2) and a second main conduction terminal (D1, D2), b) a current mirror (MC) having at least two input-output terminals (IM, OM) with said two input-output terminals (IM OM) coupled respectively to said two transistors (M1, M2) so as to supply them the bias currents, c) a voltage generator (VG) connected between said two control terminals (G1, G2), and d) a feedback path (FP) between said control terminals (G1,G2) and one (OM) of said input-output terminals, and characterised in that said voltage generator (VG) comprises a two-terminal circuit element (B1) and a bias network (MB, IG, GND) imposing on said two-terminal circuit element (B1) a constant predetermined current (IG) and said output (OT) is coupled to one (G2) of said control terminals to detect a value of the potential on said one control terminal (G2) and to determine said threshold voltage. Circuit in accordance with claim 4 realized by means of MOS transistors operating in saturation conditions. Circuit in accordance with claim 4 in which said first terminals (S1, S2) are connected to a reference potential (GND) and in which the second terminal (D1) of one (M1) of said two transistors is connected to the input terminal (IM) of said mirror (MC) and in which the second terminal (D2) and the control terminal (G2) of the other (M2) of said two transistors are connected together to the output terminal (OM) of said mirror (MC) and in which the voltage generator (VG) is connected in such a manner that the potential of the control terminal (G2) of said other transistor (M2) is less than the potential of the control terminal (G1) of said one transistor (M1). Circuit in accordance with claim 4 in which said two-terminal circuit element (B1) is connected between said two control terminals (G1, G2) and in which said bias network comprises a current generator (IG) connected between the control terminal (G2) of one (M2) of said two transistors and a first reference potential (P1) and a bias transistor (MB) having its control terminal (GB) coupled to one (OM) of said input-output terminals and a main conduction path connected between the control terminal (G1) of the other (M1) of said two transistors and a second reference potential (P2). Circuit in accordance with claim 4 also comprising a voltage divider (VD) having an intermediate output (E3) and a first (E1) and a second (E2) end terminals and comprising said two-terminal circuit element (B1) and another two-terminal circuit element (B2) preferably equal and in which said first terminals (S1, S2) are coupled to a third reference potential (P3) and in which the second terminal (D1) of one (M1) of said two transistors is coupled to the input terminal (IM) of said mirror (MC) in which the second terminal (D2) of the other (M2) of said two transistors is coupled to the output terminal (OM) of said mirror (MC) and in which the control terminal (G2) of said other transistor (M2) is coupled to said intermediate output (E3) and the control terminal (G1) of said one transistor (M1) is coupled to said first end terminal (E1) and in which said first end terminal (E1) is coupled to a second reference potential (P2) for bias of said divider (VD) and is coupled to said output terminal (OM) by a transistor (MB) held in saturation, and in which said output (OT) is coupled to said second end terminal (E2). Circuit in accordance with claim 8 also comprising a current generator (IG) connected between said second end terminal (E2) and a first reference potential (P1) and preferably at the same potential as said third reference (P3) and comprising a bias transistor (MB) having its control terminal (GB) coupled to said output terminal (OM) and main conduction path respectively connected between said first end terminal (E1) and said second reference potential (P2). Circuit in accordance with claim 4 or 8 in which said two-terminal circuit element (B1, B2) is a resistor. Circuit in accordance with claim 4 comprising in addition: a) at least a third (M7) and a fourth (M8) transistors of the same type having respectively two control terminals (G7, G8) and having a threshold voltage essentially equal to said two transistors (M1, M2) with the control terminal (G7) of said third transistor (M7) coupled to one of said control terminals (OT) of said two transistors, and b) another current mirror (MC2) having at least one input terminal (IM2) and one output terminal (OM2) with said input terminal (IM2) coupled to said third (M7) transistor and said output terminal (OM2) coupled to said fourth (M8) transistors so as to supply to them the bias currents, in which said output (UT) is coupled to the connection of the control terminal (G8) of said fourth transistor (M8) and of the output terminal (OM2) of said other mirror (MC2). Circuitry system comprising: a) an operating circuit block, b) at least one threshold voltage extraction circuit in accordance with one of claims 4 to 11 and having one output, and c) at least one bias network having an input connected to said output of the threshold voltage extraction circuit and having an output connected to said operating circuit block to supply bias currents and/or voltages.

Inventors: BRUNO DARIO, GIACALONE BIAGIO, GNUDI ANTONIO, MANARESI NICOLO, BRUNO, DARIO, GIACALONE, BIAGIO, GNUDI, ANTONIO, MANARESI, NICOLO, Manaresi, Nicolò
Applicants: CONS RIC MICROELETTRONICA, CO.RI.M.ME.
Application year: 1994
Main classification: G05F3
IPC codes: G05F3